1. Field of the Invention
This invention relates to the purification of acetic acid and relates more particularly to the purification of acetic acid resulting from the low water catalytic , carbonylation of methanol.
2. Description of the Prior Art
Various methods have been employed for producing acetic acid including, for example, the oxidation of acetaldehyde, the oxidation of petroleum naphtha, butane or the like, or the direct synthesis of methanol and carbon monoxide. One of the more useful commercial methods for the production of acetic acid is the carbonylation of methanol as disclosed in U.S. Pat. No. 3,769,329. The carbonylation catalyst comprises rhodium, either dissolved or otherwise dispersed in a liquid reaction medium or else supported on an inert solid, along with a halogen-containing catalyst promoter as exemplified by methyl iodide. The rhodium can be introduced into the reaction system in any of many forms, and it is not relevant, if indeed it is possible, to identify the exact nature of the rhodium moiety within the active catalyst complex. Likewise, the nature of the halide promoter is not critical. A large number of suitable promoters are disclosed, most of which are organic iodides. Typically, the reaction is conducted with the catalyst being dissolved in a liquid reaction medium through which carbon monoxide gas is continuously bubbled.
An improvement in the prior art process for the carbonylation of an alcohol to produce the carboxylic acid having one carbon atom more than the alcohol in the presence of a rhodium catalyst is disclosed in copending, commonly assigned application U.S. Ser. No. 699,525, filed Feb. 8, 1985; European Patent Application No. 161,874, published Nov. 21, 1985; and U.S. Ser. No. 870,267, filed Jun. 3, 1986. As disclosed therein, acetic acid (HAc) is produced from methanol (MeOH) in a reaction medium comprising methyl acetate (MeOAc), methyl halide, especially methyl iodide (MeI), and rhodium present in a catalytically-effective concentration. The invention therein resides primarily in the discovery that catalyst stability and the productivity of the carbonylation reactor can be maintained at surprisingly high levels, even at very low water concentrations, i.e., 4 wt. % or less, in the reaction medium (despite the general industrial practice of maintaining approximately 14 wt. % or 15 wt. % water) by maintaining in the reaction medium, along with a catalytically-effective amount of rhodium, at least a finite concentration of water, methyl acetate and methyl iodide, a specified concentration of iodide ions over and above the iodide content which is present as methyl iodide or other organic iodide. The iodide ion is present as a simple salt, with lithium iodide being preferred. The applications teach that the concentration of methyl acetate and iodide salts are significant parameters in affecting the rate of carbonylation of methanol to produce acetic acid especially at low reactor water concentrations By using relatively high concentrationsof the methyl acetate and iodide salt, one obtains a surprising degree of catalyst stability and reactor productivity even when the liquid reaction medium contains water in concentrations as low as about 0.1 wt. %, so low that it can broadly be defined simply as "a finite concentration" of water. Furthermore, the reaction medium employed improves the stability of the rhodium catalyst, i.e., resistance to catalyst precipitation, especially during the product-recovery steps of the process wherein distillation for the purpose of recovering the acetic acid product tends to remove from the catalyst the carbon monoxide which in the environment maintained in the reaction vessel, is a ligand with stabilizing effect on the rhodium. U.S. Ser. No. 699,525 and U.S. Ser. No. 870,267 are herein incorporated by reference.
The acetic acid which is formed by the carbonylation of methanol is converted to a high purity product by conventional means such as by a series of distillations. While it is possible in this manner to obtain acetic acid of relatively high purity, the acetic acid product contains a considerable amount of by-product impurities, determinable on the basis of their reducing action on permanganate. The amount of such reducing impurities is referred to as the permanganate time. Since the permanganate time is an important commercial test which the acid product must meet for many uses, the presence therein of such impurities is highly objectionable. Apparently, the removal of minute quantities of these impurities by conventional rectification alone is difficult since the impurities distill over with the acetic acid.
Among the residual impurities which have been found to degrade the permanganate time are alkyl iodide impurities which are most likely carried over into the product stream from the catalyst solution in the reactor. Also found in the acetic acid product are various unsaturated and carbonyl impurities including crotonaldehyde, ethyl crotonaldehyde and the 2-methyl-2-pentanal isomer thereof. As has been previously stated, it is both difficult and costly to remove the iodides, unsaturates and carbonyl impurities from the acetic acid product by physical methods since such impurities are present in such minute amounts. Accordingly, an economical process for removing such impurities is needed.
Various methods have been suggested to purify or remove nonacidic components from carboxylic acids. For example, U.S. Pat. No. 4,576,683 discloses a method of separating C.sub.1 -C.sub.10 aliphatic and C.sub.3 -C.sub.10 olefinic carboxylic acids from mixtures with nonacids by extractive distillation using an amide as an extractant to recover an extractant-acid mixture by rectification. The method disclosed in the patent is described as being particularly suitably applied on aqueous mixtures of formic, acetic and/or propionic acid which mixtures contain unconverted hydrocarbons and other oxygenated compounds such as mixtures with alcohols, aldehydes and/or ketones and which may also contain further contaminants such as effluents from the tent are selected from lactams having 5 or 6 membered rings Pyrrolidone and derivatives thereof are specifically disclosed.
U.S. Pat. No. 4,268,362 is concerned with providing a method of removing formaldehyde from raw acetic acid which has been formed by synthetic reactions such as oxidation of acetaldehyde, gas phase or liquid phase oxidation of butane, oxidation of petroleum naphtha or paraffins, as well as the reaction of methanol with carbon monoxide. The separation process involves treating the acetic acid in a heating zone at a temperature at about the boiling point of the acetic acid or higher, removing the heated product and delivering it to a distillation zone and operating the distillation zone so as to obtain a lower boiling fraction, a higher boiling fraction and an intermediate acetic acid fraction which will have a formaldehyde content of 300 ppm or lower.
U.S. Pat. No. 3,725,208 is concerned with a process for the removal of small amounts of aldehyde impurities from acrylic acids which comprises adding to the acrylic acid minor amounts of a compound selected from the group consisting of sulfuric acid, hydrazine, phenyl hydrazine, aniline, monoethanolamine, ethylene diamine and gylcine and subjecting the acrylic acid mixture to distillation. Although hydrazine usually reacts exothermically with acrylic acid to form pyrrazolidone, and amines such as monoethanolamine and ethylene diamine have the properties of forming salts and aminocarboxylic acids with these compounds react predominantly with aldehydes contained in acrylic acid and can remove them from the acrylic acid.
Japanese Patent Application 60-222439 discloses purification of acetic anhydride produced by the ketene process in which acetic acid is thermally cracked to ketene which then combines with acetic acid through an absorption reaction to produce acetic anhydride. The impurities contained in acetic anhydride produced in this manner are many low and high boiling compounds present at the time when acetic acid is thermally cracked and when acetic acid and ketene are reacted. However, the exact nature of the impurities contained in the acetic anhydride are not disclosed. Treatment with ozone gas in the absence of an oxidation catalyst was found to provide a quality product equal to or greater than that produced in purification by distillation.
Japanese Patent Publication 55(1980)-64545, published May 15, 1980, discloses purification of acetic acid in which an ozone-containing gas is introduced to the acetic acid in the absence of an oxidation catalyst to obtain acetic acid of a higher quality as measured by a potassium permanganate test and sulfuric acid colon test. The identities of the impurities contained in acetic acid are not identified.
U.S. Pat. No. 3,928,434 to Saunby discloses reducing the content of oxidizable impurities in acetic acid produced by hydrocarbon oxidation by treating the acetic acid with oxygen in the presence of a transition metal compound to oxidize unsaturated ketones. Saunby, in col. 1, lines 45-60, points out that, heretofore, oxidizable impurities can be destroyed by reaction with ozone, but that such treatment suffers the drawback of the risk involved in handling ozone at elevated temperatures in organic liquid. The Saunby disclosure is directed to removal of alpha, beta-unsaturated ketone impurities in acetic acid produced by hydrocarbon oxidation.